What is a Wave?
A wave is a common term for the number of different ways in which energy (kinetic and potential energy including heat, light, force, charge, etc.) can be transferred between systems via oscillations in a medium (empty space or objects with mass like molecules) in a wave-like manner.
Traveling Waves: Crash Course Physics #17. This video discusses waves using a string to explain the properties of a wave and how it transfers energy.
All Waves Transfer Energy
All waves (including sound waves, electromagnetic waves, and even water waves) transfer energy, and all energy is transferred in a wave-like motion.
All energy traveling between systems travels in a wave-like motion via vibrations (either in fields or molecules); the faster the vibration, the stronger the wave.
Even gusts of wind or the energy you burn when you workout can be described in terms of waves. The fundamental building blocks of the universe have wave-particle duality, including the four forces, so it only makes sense that waves play an important role in the exchange of energy in all systems.
With the above said, there are two very different types of waves, one that moves through free space and one that moves through a medium.
Below we will discuss other details including how water waves, gravitational waves work, other waves transfer energy. First, this video from Bozeman Science is a probably one of the better wave videos online.
PS4A – Wave Properties
The Types of Waves That Transfer Energy
There are two main types of waves that transfer energy: 1. mechanical waves, which require a medium, are continuous, and can be transverse or parallel, and 2. quantum waves, which travel in free space, are quantized, and can only be transverse.
- Mechanical waves (like sound) transfer energy via vibrations in a given medium. For instance, sound vibrates air molecules. The waves, in this case, aren’t “a thing;” they are a disruption of a medium. All mechanical waves transfer energy in a continuous motion. They can travel parallel to the source and thus be “longitudinal,” or they can travel perpendicular to the source and be “transverse,” depending on the situation.
- Electromagnetic waves (like light) and other Quantum waves do not require a medium, and are themselves “a thing;” they are the building blocks of the standard model (vibrations in their respective quantum field). Electromagnetic waves transfer energy via electric and magnetic fields which propagate perpendicular to one another (they are transverse). Electromagnetic waves are pure massless kinetic energy being transferred across free space in a wave-like motion. A given medium will slow down and reflect these types of waves depending on their wavelength and the molecular structure of the medium. Other quantum waves have similar but different behavior (see wave-particle duality). All quantum waves are “quantized” (they aren’t continuous, they jump to discrete states) and are always transverse (propagate perpendicular to their source).
How Do Different Types of Waves Transfer Energy?
- In electromagnetic waves, energy is transferred through vibrations in the electromagnetic field (see what is electromagnetic energy). Electromagnetic waves are pure kinetic energy but can add to the potential energy (relative mass) of a system (for instance if an electron absorbs a massless photon it gains potential energy).
- In sound waves, energy is transferred through the vibration of air particles, or the particles of other mediums through which sound can travel (like water or a solid). With sound, kinetic energy is transferred from molecule to molecule, vibrating the molecule out of its normal position and back, oscillating between kinetic and potential energy as the sound travels through the medium.
- In water waves, energy is transferred through the vibration of the water molecules. Water works a little differently than sound (because water is itself a medium which mechanical waves are traveling through). That aside, the water molecules work just like air molecules, oscillating between kinetic and potential energy, transferring energy through the water and moving the water along with it to create the waves of the ocean. Since water molecules are more massive than air, we can see waves moving and displaying nearly all the different wave types in nature (even reflecting light, allowing sound to travel through its medium, and being affected by gravity).
- Gravitational waves transfer energy just like sound waves and water waves, except gravitational waves transfer energy through “spacetime” (so “spacetime” is the medium).
- All quantum particles, at their core, can be described as waves (vibrations in a quantum field). Thus all physical objects can be described, on a fundamental level, by their wave-like properties (quantum field theory and string theory).
If you clap your hands right now, since energy is never created or destroyed, you will be exchanging a bit of potential energy in your body for a nice sound wave that travels through the molecules in the air via vibrations in those molecules to your ears. You will not, however, be touching your hands, as two things can never truly touch (Pauli exclusion).
Have Gravitational Waves Been Discovered?!? | Space Time | PBS Digital Studios
TIP: Waves transfer energy, but they don’t transfer rest-mass (the true mass of a system). They can, however, transfer relative-mass (an object’s mass counting kinetic energy). Learn more about the types of mass or see mass-energy equivalence.
A Wave is Different Than a Particle
Wave is a broad common language term that can apply to anything with wave-like behavior: a wave on a string, an acoustic sound wave traveling through water, a water wave, a seismic wave, a shock wave, a wind wave, etc.
Sometimes we are talking about the wave-like behavior of a medium (like air or water), and sometimes about the underlying oscillation of the medium and the transfer of energy. With that in mind, in the purest sense, it is the underlying oscillation and not the wave-like behavior of the medium that we are discussing when we say “wave”, as the underlying oscillation and transfer of energy in a wave-like motion is what causes the wave-like behavior of the medium itself!
We can sum this up by saying, “a wave is different than a particle.” The molecules in a medium mostly stay in place, they don’t travel with the wave in a medium, instead what we see as waves in a medium like water is energy being transferred from molecule to molecule distorting and disturbing the medium.
FACT: Whether we see a light, or we hear a sound, our senses are picking up vibrations. The frequency of the vibration determines the wavelength, which determines how we perceive the energy.
How To Measure the Energy of a Wave
Measuring the energy of each type of wave works a bit differently but in general the faster the vibration, the stronger the wave.
First, you’ll need a quick refresher on the properties of a wave:
The properties of a wave:
- Wavelength: The distance between crests
- Frequency: The number of wave crests that pass a point every second.
- Velocity: the speed and direction that the wave crest itself is moving.
- Amplitude: The height of the wave crest above the undisturbed position.
Now here are some examples on how to measure the energy of a given wave:
- For water waves, high speed and long wavelength (like a tsunami) have the most energy.
- For electromagnetic waves, speed is constant (light speed in a vacuum), so waves with a high frequency and a short wavelength (like X-rays) are the most energetic.
- For all waves, a greater amplitude means more energy.